This invention relates to the storage of digital data and, more particularly, to a method for compactly storing a series of digital data entries which are representative of the occurrence of a large number of random events.
The frequency of a periodic signal is typically measured by counting the number of events (pulses, zero crossings, etc.) that occur over some fixed sample period, T. If a number, C, of events are counted over a sampling interval, the measured frequency, F, of the signal is simply calculated by the equation: F=C/T.
This equation works well for a periodic signal, but when the number of event occurrences is varying statistically, and the number of events, C, during a sample time interval, T, is small, then inaccuracies result. The error in the measurement of frequency can be reduced to an arbitrarily small value by increasing the length of the sample interval, to a sufficiently large value. Alternatively, successive sample intervals can be averaged to produce a calculated frequency that is close to the true mean value.
This method works well as long as a sufficient number of inputs are used in the average to offset the effects of statistical variations. At low levels of occurrences, this number is quite large and at high levels this number becomes substantially reduced. The exact number of readings needed to achieve a specified accuracy can be calculated using standard statistical methods. In particular, if the random process can be modeled by a Poisson distribution, as is the case for neutron activity in a nuclear reactor, then the number of samples, N, required to meet a specified accuracy at least 95% of the time (two standard deviations) is: N=4/.lambda.t.epsilon..sup.2 where, .lambda. is the mean number of counts occurring in a one second interval, t is the basic sampling period in seconds, and .epsilon. is the specified relative error.
The response time of a measuring instrument to a change in frequency of incoming pulses is directly related to the number of input samples used to calculate pulse frequency. If N sampls are used in the calculation, then the response time of the instrument will simply be N sample times. Therefore, accuracy and response time can be traded off in a straightforward manner.
Within a nuclear reactor, neutron activity is an indication of power being generated by the reactor. An indication of both the power and the rate of change of power within the reactor is important for both control and safety systems. The rate of change of neutron activity is usually indicated as the exponential rate of change (typically in decades per minute). The equation for the exponential rate of change of neutron activity is given by: EQU Rate=(log (F.sub.2)-log (F.sub.1)/T or (1) EQU Rate=(log (F.sub.2 /F.sub.1))/T (2)
where F.sub.1 and F.sub.2 are consecutive measurements of the frequency of neutron activity and T is the sample interval between the two frequency measurements. The resulting rate has units of decades per sample interval.
An instrument, typically referred to as the source range instrument, designed to cover the range of neutron activity from a few pulses per second to millions of pulses per second, must meet two general design criteria with respect to the measurement of both the frequency of neutron activity (level) and the rate of change in frequency of neutron activity (rate). First, to provide the required accuracy and stability of measurement, the instrument must use a sufficiently long sampling interval so that the variations in the reading due to the random nature of neutron activity are minimized. Second, to provide adequate response time, the instrument must provide an accurate, stable reading in a timely fashion. The response time requirements generally vary over the operating range of the instrument from tens of seconds at the low end to one second or less at the upper operating range. Different response times are also generally specified depending on the type of input presented to the instrument. For example, the response time requirements may be separately specified for exponetially increasing inputs and step change inputs.
In addition, the instrument may also be required to distinguish between exponential changes in inputs and step changes in inputs. This feature may be required so that the small step change in the input does not produce an indication of an abnormally high rate of change that may needlessly cause a trip in the reactor.
To perform the required level and rate calculations, historical neutron activity data is required. If for example, x.sub.1, x.sub.2 . . . , x.sub.16,640 represent the number of counts received during the most recent 100 millisecond sample period, the next most recent 100 millisecond sample period, and so on to the oldest sample, then an estimate of the mean value of the level, L is calculated from the equation: EQU L=10(x.sub.1 + . . . x.sub.N)/N (3)
where L has units of counts per second.
The value of N (and thus the effective sampling time of the instrument for the level measurement) may be dynamically adjusted to meet the required accuracy response time and statistical stability of the source range instrument.
For the frequency calculation, the rate is calculated according to the equation: EQU R=(600/M) log (S.sub.1 /S.sub.2) (4)
where S.sub.1 is the sum of the last M historical terms (1 to M) and S.sub.2 is the number of the next M historical terms (M+1 to 2M) where M is the number of terms used for each summation. With a sample period of 100 milliseconds, R will have units of decades per minute. Here again, the value of M (the effective sampling time of the instrument for the rate measurement) may be dynamically adjusted to meet the required accuracy, response time and statistical stability of the source range instrument.
To meet the specified accuracy and stability requirements for the instrument, a large number, for example at least 15,000, of historical data samples must be stored to perform the required calculations. In order to eliminate the need for excessive memory in the instrument, a method for storing this data in a compact manner is required.